Injection Molded Soldering (IMS) is a process with many applications, primarily suited for low-cost solder bumping of semiconductor wafers. The process basically involves scanning a head which dispenses molten solder through a linear slot over a mold plate to fill cavities therein with molten solder. After the scan, the solder in the cavities is solidified and then the mold plate is aligned to and placed in contact with a wafer by an appropriate fixture. This assembly is then heated to reflow and transfer the solder from the mold plate cavities to metallized pads on the wafer. After cooling and separating the wafer and mold plate, the wafer is bumped with solder preforms typically used for flip chip applications.
Solder transfer from mold plate to wafer in IMS wafer bumping is routinely practiced using flux as the activating agent. Using flux, however, is not ideal for every application and there may be a number of problems associated with the use of flux. First, there are some applications such as those cited in references [U.S. Pat. No. 5,604,831] and [U.S. Pat. No. 5,852,257] that cannot tolerate the use of flux. These include optical applications that would suffer damaging contamination to sensitive optical components if flux were included in any process steps. Secondly, extra processing steps are required to apply flux for solder transfer and reflow as well as cleaning flux from both wafers and mold plates after processing. These steps increase cost and complexity for IMS wafer bumping, and may shorten mold life due to extra processing steps. Third, if the flux application is accomplished by a wiping over the filled mold plate, there is the chance that some of the filled cavities might lose their solder volumes. This is due to the surface tension of the liquid flux causing a “suctioning effect” over the cavities as it is applied. Thus bumping yield could be reduced. Fourth, using flux creates the possibility of residual contamination left behind after cleaning. Some of these chemical residues may adversely affect long-term reliability of the entire solder interconnect structure.
There thus exists a need to eliminate the use of flux for solder transfer and reflow processing done with IMS.